Study On The Respone Of Cladoceran To Environment Change Over The Past 2000 Years In Lake Gonghai,Shanxi

Alpine lakes are usually far away from direct disturbance of human activities,which are sensitive to environment changes and can provide high-resolution records of ecological changes.Thus,these lakes are ideal platform for paleolimnological studies for the interpretation of global environmental changes.Chinese Loess Plateau is located in North China,where has rare small freshwater lakes.However,most of them are in a stage of rapid degradation.At present,there is no systematic study on the lakes ecological changes in this area.For a clear understanding on the lakes ecological changes in Chinese Loess Plateau,we select Lake Gonghai which is an alpine lake,to examine ecological changes over past ~2000 years and ~200 years using cladocera,chlorophyll a,isotope and geochemical refers,as well as climate data and the atmospheric N deposition data.The main conclusions are as follows:Planktonic cladoceran(mainly Bosmina longispina)are the dominant species before 900 AD.After 900 AD,the percentage of planktonic taxa decreased until it almost disappeared;Chydorus sphaericus,which is principally associated with shallow or littoral habitats and frequently enters the planktonic zone to utilize pelagic resources,became the dominant species in Cladocera community.In recent decades,planktonic Bosmina(mainly Bosmina longirostris and a small number of Bosmina longispina)appeared and increased continuously.Over the past 2000 years,the vegetation in the catchment has become the main driver for the change of Cladocera assemblage by adjusting the food availability through enhancing the organic matter content and nutrients inputs.Before 900 AD,the vegetation condition was well,resulting in thatmore organic matter and nutrients entered the lake.Vegetation condition increased food availability by providing food directly to cladocera and indirect through the nutrition transfer way of nutrient-algae-cladocera.Increasing food availability in the open water area wasbeneficial to the growth of planktonic cladocera.After 900 AD,the vegetation condition was degenerated,which decreased nutrients inputs of the lake.Limited food availability made planktonic cladocera lose the competitive advantage,while Chydorus sphaericus became the dominant species.Temperature has been an important driver of influencing in cladocera over the past 200 years.Before 1967,temperature had little effect on cladocera assemblages in Lake Gonghai.Planktonic species Bosmina,which prefer warmer temperature environment,began to increase in Lake Gonghai from 1967.Climate warming prolonged the growing period,shorted the duration of ice-cover,increased water temperature and food availability,and thus directly and indirectly impacts the cladoceran composition.Over the past 2000 years,cladocera and diatoms have responded differently to climate change.Climate change mainly affects cladocera community composition by regulating Cladocera food availability through the development of vegetation in the catchment.While diatom community composition is mainly affected by the supply of nutrients(phosphorus)into the lake and the mixing of lake water.Under the background of global warming,cladocera and diatom assemblages responded rapidly to the warming in 1960 s.The change of cladocera assemblages was mainly due to the changes of growth period,habitat and food availability.Diatom was mainly due to the aggravation of lake stratification caused by global warming,and smaller diatom replaced larger diatom as dominant species.It shows that the response mechanism of lakes to climate change is different from that of natural condition and global warming.Because no signal of atmospheric nitrogen deposition was recorded in the sediments of alpine lakes and the low atmospheric nitrogen deposition rate in this area,it shows that atmospheric nitrogen deposition has not become the main driver of lake ecological change until now.